One known form of developing device installed in an image forming apparatus such as an electrostatic copying apparatus comprises a shutter mechanism 24 in the interior of a main body 21 of the developing device as shown in FIG. 3. The shutter mechanism 24 is designed such that the operation thereof can be switched thereby to feed developer 22 stored in the main body 21 to a developing magnet roller 23 or to suspend the supply mentioned above. At the upper end of the main body 21 is disposed a toner hopper 28 comprising a toner feed roller 30 therein, and the space underneath the toner feed roller 30 is interconnected with the space above the shutter mechanism 24 thereby forming a toner feed path 31.
In the aforementioned developing device, the shutter mechanism 24 is open at the time of performing development as shown in FIG. 3, and the developer 22 which has been fed through the shutter mechanism 24 is adhered to the peripheral surface of a rotative sleeve 23a of the developing magnet roller 23 thereby forming a magnetic brush. The magnetic brush formed by the toner component of the developer 22 is brought in contact with and rubbed against the surface of a photoreceptor belt 25 and thereby toner is continuously supplied onto the photoreceptor belt 25.
During the time in which development is not performed, the shutter mechanism 24 is closed as shown in FIG. 4, whereby the supply of the developer 22 to the developing magnet roller 23 is suspended. Even during this time period, the rotative sleeve 23a of the developing magnet roller 23 and a rotative sleeve 26a of a delivery magnet roller 26 disposed above the developing magnet roller 23 are continuously rotating, and the developer 22 is conveyed accordingly into the space above the shutter mechanism 24 and gradually accumulates localizing in the space which extends from the delivery magnet roller 26 to the shutter mechanism 24. The developer 22, which has been conveyed by the delivery magnet roller 26 during the time in which development is not performed as shown in FIG. 4, accumulates in the space which extends from the delivery magnet roller 26 to the shutter mechanism 24, resulting in forming a heap above the delivery magnet roller 26 and coming in contact with the upper wall portion of the main body 21. This heap of developer 22 confines the air in the space in the vicinity of the toner feed path 31 within the main body 21.
Thereafter, the remaining developer 22 around the developing magnet roller 23 is conveyed upward by the developing magnet roller 23, causing the developer 22 which has been conveyed by the delivery magnet roller 26 to press the heap of developer 22 above mentioned. Then, the air confined in the above space is gradually compressed developing a relatively high pressure. Loosing a way of escape within the main body 21, the air thus compressed flows all at once into the toner hopper 28 from the inside of the main body 21 through the toner feed path 31, as indicated by the arrow X of FIG. 4.
When the air comes into the toner hopper 28, toner stored within the toner hopper 28 is blown off by the air. This tends to cause air including a great amount of toner to go out through a gap between the toner hopper 28 and its lid 28a as indicated by the arrow Y of FIG. 4 and to scatter outward. The toner which has been out and scattered from the toner hopper 28 then adheres to a corona charger, members in an optical system and other members disposed in the copying apparatus or the like, resulting in considerable deterioration of the image quality.
In order to overcome the above problem, such a method can be adopted that the air which has been compressed in the main body 21 is positively released outside the developing device while a filter or the like is attached to the outlet of the air for preventing the discharge of toner. However, this method requires periodic replacement of the filter or the like bringing about another problem such as a considerable increase in the maintenance cost.